A bill set to be introduced in the Massachusetts Legislature Friday would allow self-driving cars on public roads, but impose a mileage-based tax on their use, allow some large municipalities to ban them, and require all such cars to be zero-emissions vehicles.

The measure, sponsored by Representative Tricia Farley-Bouvier, a Pittsfield Democrat, and Senator Jason Lewis, a Winchester Democrat, is the latest attempt to enact statewide rules for autonomous vehicles after several similar bills failed to advance last year.

Self-driving cars now fall in a legal gray area in Massachusetts — state law doesn’t explicitly ban or allow them. With numerous companies in a race to develop and commercialize the technology, lawmakers are worried about falling behind.

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“We want to encourage innovation around new transportation options like autonomous vehicles,” Lewis said. “We want to maximize the potential benefits in a way that protects public safety.”

Governor Charlie Baker and Boston Mayor Martin J. Walsh have each issued executive orders encouraging the testing of driverless vehicles. But Baker mandated that the vehicles be able to pass state inspection, meaning they need to have brake pedals and steering wheels.

In contrast, the new bill would allow companies to deploy fully autonomous cars without traditional controls, as long as they have a hard-wired switch that can be used to force the car to pull over and stop.

Passenger cars would be required to have someone inside during operation; only freight and emergency vehicles would be permitted to travel more than one mile without a passenger. The provision, Lewis said, is intended to prevent a proliferation of “zombie cars” driving in circles while, for example, their owners shop in areas with limited parking.

Except for noncommercial test vehicles, the bill would ban cars that typically drive themselves, but require that their operators be prepared to take control at any moment. Lewis said such systems “go against human nature” by requiring drivers to constantly pay attention to the road without doing anything.

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Because zero-emission vehicles do not generate gasoline taxes, Lewis and Farley-Bouvier propose taxing them with a “road usage charge” of at least 2.5 cents per mile. Cars with multiple passengers, or that travel during off-peak hours, or drive in areas with little public transit could be eligible for discounts. Higher rates could be charged to heavier autonomous vehicles and those operating in congested areas or with only one passenger.

The bill also would require self-driving cars to:

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Be clearly marked as an autonomous vehicle.

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Drive slowly in thickly settled areas and school zones.

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Be zero-emissions vehicles — running on batteries or hydrogen cells — unless they weigh more than 8,500 pounds.

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Meet federal safety standards.

In addition, the bill would allow densely populated municipalities to ban or limit private autonomous vehicles that don’t provide public transportation. Ride-hailing firms such as Uber, however, could deploy self-driving cars.

Lewis said that Baker and legislative leaders have yet to weigh in on the bill, and acknowledged it was unlikely to be approved in its current form. But he said it was important to begin a discussion about how to regulate the rapidly developing field.

Bryan Reimer, associate director of the New England University Transportation Center at MIT, agreed the state needs a clear law governing autonomous vehicles. But he said the proposed legislation has several flaws. Chief among them, he said, is that it doesn’t require operators to report accidents or other incidents, as California does.

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Reimer also said singling out autonomous cars for a zero-emissions requirement and mileage tax would discourage companies developing such vehicles from investing in Massachusetts. Even gas-powered self-driving cars could benefit the environment, he noted, if they are shared by multiple users.

“We shouldn’t be linking these policies to autonomous vehicles, because we don’t know enough yet about the potential long-term deployment structures of these systems,” Reimer said.

Earlier this month, Cambridge-based startup nuTonomy Inc. began the first public trial of a self-driving car in Massachusetts in Boston’s Seaport district.